Large Aperture solar telescope using a modded Lunt etalon?

Frankenscope? Let's see it!***be advised that NOTHING in this forum has been safety tested and you are reading and using these posts at your own peril. blah, blah, blah... dont mess around with your eyesight when it comes to solar astronomy. Use appropriate filtration at all times...
poita

Re: Large Aperture solar telescope using a modded Lunt etalon?

Post by poita »

That is what I figured, but am confused as to why the higher end scopes use APO type systems?

Another question, why the two ERFs? does the mounting of the front ERF behind the objective lead to too much energy getting through and requiring the second ERF?


User avatar
Merlin66
Librarian
Librarian
Posts: 3970
Joined: Tue Dec 13, 2011 1:23 pm
Location: Junortoun, Australia
Has thanked: 173 times
Been thanked: 615 times
Contact:

Re: Large Aperture solar telescope using a modded Lunt etalon?

Post by Merlin66 »

I think the APO's are used because the users have them!
There's only one "main" ERF and that's the one you place up front...the smaller "mini-ERF" can be left in place to give "belt and braces" protection to the sensitive BF filter...
( I and others have removed the "mini-ERF" from the BF10/BF15 when it's used behind a Baader D-ERF in a PST Mod...)


"Astronomical Spectroscopy - The Final Frontier" - to boldly go where few amateurs have gone before
https://groups.io/g/astronomicalspectroscopy  
http://astronomicalspectroscopy.com
"Astronomical Spectroscopy for Amateurs" and
"Imaging Sunlight - using a digital spectroheliograph" - Springer
PEterW
Ohhhhhh My!
Ohhhhhh My!
Posts: 151
Joined: Fri Apr 27, 2012 8:57 pm
Has thanked: 1 time
Been thanked: 6 times

Re: Large Aperture solar telescope using a modded Lunt etalon?

Post by PEterW »

Need to replace the focusser for a 10:1 microfocusser you'll never be able to get perfect focus. I had one.... Regret setting given the current discussions!

Cheers

PEter


brianb11213

Re: Large Aperture solar telescope using a modded Lunt etalon?

Post by brianb11213 »

Need to replace the focusser for a 10:1 microfocusser you'll never be able to get perfect focus.
Well, the single speed focuser in a fast scope makes accurate focusing harder than it needs to be, and focusing is difficult enough at the best of times ... but I (just about) managed to focus my 80mm f/6 with a single speed focuser exven with a 4x Powermate hung behind it when there was no other option.

As for a 120mm f/5 refractor as a basis for a Ha scope: I'd be somewhat sceptical on the basis that the fast refractors I've seen in the 120mm / 150mm class have tended to have optics which were not all that great, even allowing for the fact that chromatic aberration isn't an issue for solar work. Can't see the point in spoiling the ship for a hapennorth of tar, as the saying used to go.


poita

Re: Large Aperture solar telescope using a modded Lunt etalon?

Post by poita »

The reason I was asking is that a 120 f5 scope is available 2nd hand for $250, but if they are not very good then I guess that is why!

Maybe I should save up and go straight to a SW150.

I could always try it on the lomo 105/650 I guess.

Are there any advantages to going with a f8 or f10 or f11 scope over an f5 or f6?


colinsk

Re: Large Aperture solar telescope using a modded Lunt etalon?

Post by colinsk »

Also, it you trace the light cone for a less than f/10 objective you find that the optics of the PST etalon vignette. I am told that even at f/10 there is vignetting!


poita

Re: Large Aperture solar telescope using a modded Lunt etalon?

Post by poita »

Yes, I realised that limitation with the PST, part of the reason I chose an f11 system for the PST mod.

But is there a similar issue with the Lunt Mod? Are there any disadvantages to using say an F8 over a F6, other than length?


colinsk

Re: Large Aperture solar telescope using a modded Lunt etalon?

Post by colinsk »

Are you talking a LS60?


poita

Re: Large Aperture solar telescope using a modded Lunt etalon?

Post by poita »

Yes, an LS60, not the pressure tuned model.


User avatar
marktownley
Librarian
Librarian
Posts: 42270
Joined: Tue Oct 18, 2011 5:27 pm
Location: Brierley Hills, UK
Has thanked: 20424 times
Been thanked: 10243 times
Contact:

Re: Large Aperture solar telescope using a modded Lunt etalon?

Post by marktownley »

Yes, I realised that limitation with the PST, part of the reason I chose an f11 system for the PST mod.

Ok, I get the idea that with a PST mod if the donor scope has a focal ratio less than f10 there is vignetting. But what is the effect if the donor scope f ratio is greater than f10? Is the output from the first collimated lens still perfectly parallel ie colliamted, or because of the different f-ratios does it have a slight convergence / divergence that meand it is 'not quite' colliamted?


Image
http://brierleyhillsolar.blogspot.co.uk/
Solar images, a collection of all the most up to date live solar data on the web, imaging & processing tutorials - please take a look!
User avatar
Merlin66
Librarian
Librarian
Posts: 3970
Joined: Tue Dec 13, 2011 1:23 pm
Location: Junortoun, Australia
Has thanked: 173 times
Been thanked: 615 times
Contact:

Re: Large Aperture solar telescope using a modded Lunt etalon?

Post by Merlin66 »

The f10 lower limit is the result of the barlow lens focal length, above f10 only helps matters.
For the Lunt 60 mod, Viljo has used an f8 refactor as a donor; the optical system on the Lunt etalon shows that any donor >f4.5 will work well.
Different optics, different solutions....


"Astronomical Spectroscopy - The Final Frontier" - to boldly go where few amateurs have gone before
https://groups.io/g/astronomicalspectroscopy  
http://astronomicalspectroscopy.com
"Astronomical Spectroscopy for Amateurs" and
"Imaging Sunlight - using a digital spectroheliograph" - Springer
Photonist
Im an EXPERT!
Im an EXPERT!
Posts: 303
Joined: Sat Jan 07, 2012 5:57 pm

Re: Large Aperture solar telescope using a modded Lunt etalon?

Post by Photonist »

poita,

I would vote for a longer focal ratio, f/8 or f/10 as minimum and big aperture, but then I’m mainly interested in close-ups and hi-res. My donor scope is f/8 and the effective FD of the mod is f/14.8 (2220/150 mm). I have used a 120 mm aperture mask a few times (f/18.5) and find the contrast is a bit better. I think the improvement is not only seeing related, but also due to being better on band?

I think most systems have some vignetting, hot spots or uneven illumination. I have a mild hot spot using a Coronado BF10 (which I prefer for much shorter exposure), but not with Lunt BF1200. The spot it is easy to correct with flat frames or some Photoshop tweaks (I need flats for pollen bunnies anyway).

Perhaps a SW120/1000 mm, some 127 mm or a TS102/1100 mm if you don’t want to go for the really big SW150?

Regards,

Viljo


poita

Re: Large Aperture solar telescope using a modded Lunt etalon?

Post by poita »

I'm happy to go large, but I'm looking for the best value for my limited funds. I'm not sure how much magnification my seeing will allow either.

So higher f numbers give better contrast and obviously greater magnification for a given aperture.

No other issues?

It will really come down to what I can find that I can afford. The blocking filter is going to eat up most of my budget.
I also wanted to check that a 90mm DERF is going to be big enough.


Photonist
Im an EXPERT!
Im an EXPERT!
Posts: 303
Joined: Sat Jan 07, 2012 5:57 pm

Re: Large Aperture solar telescope using a modded Lunt etalon?

Post by Photonist »

poita,

I’m very satisfied with my mod and don’t have any big issues. However, there is little latitude for turning the tuning wheel, a PT version would perhaps be nice. But I think I have it quite well on band so no problem, I don’t feel there is a need to adjust it daily… so far.

Another small issue (or an interesting topic) is the poor transmission of the Lunt BF1200 compared to the Coronado BF10. The exposure time is for example 6 ms and 1.5 ms respectively. That’s too much for me. I hope someone could elaborate on the differences of these BF’s and how to perhaps mod the Lunt BF

The SW120 is quite affordable and seem to work very well (with a PST, see Cai-Uso’s images). It costs in Europe around 300 Euros and the SW150 around 580 Euros and the Elite 127 about 500 Euros at http://www.teleskop-express.de/shop/ind ... rture.html.

In my mod the 110 mm ERF is 345 mm behind the lens. A 90 mm ERF would be about 496 mm behind the lens so I think it would work well also.

Regards,

Viljo


colinsk

Re: Large Aperture solar telescope using a modded Lunt etalon?

Post by colinsk »

In trying to mod the Lunt BF the first thing I would check is the reflectivity of the mirror. I was told before production started that three mirrors were going to be made. 30%, 40% and 50% reflective and the choice of which one to use would be made on a etalon to etalon basis. I don't have any idea if this plan was ever implemented but I do have reason to suspect that the diagonal mirror is not designed to be 90% reflective.


brianb11213

Re: Large Aperture solar telescope using a modded Lunt etalon?

Post by brianb11213 »

In trying to mod the Lunt BF the first thing I would check is the reflectivity of the mirror. I was told before production started that three mirrors were going to be made. 30%, 40% and 50% reflective and the choice of which one to use would be made on a etalon to etalon basis. I don't have any idea if this plan was ever implemented but I do have reason to suspect that the diagonal mirror is not designed to be 90% reflective.
Ummm. Unless the diagonal version of the blocking filter uses a dielectric coating as part of the filter .... but why would they do that? In any case Lunt sells "straight" blocking filters which clearly don't have a mirror inside at all, lossy or otherwise.

Funny thing about my Solarscope SF-100 BF. That's a straight through type too - loks like a 2" extension tube, adds about an inch of length. I haven't attempted to disassemble it (why should I, it's working just great) but it appears to have a lot of glass in it, there's a good inch between the inner surface (which appears to be a bluish green glass surface) and the outer surface (which has a shiny peach coloured coating). When you look through the thing, on its own, it's surprisingly transmissive (probably about 10%), imparting a very slightly greenish blue hue to objects on the other side. I presume it's sort of minus red with a narrow band notch in the bandpass to pass the h alpha line & a nanometer or two either side.

Ken Huggett who makes the Solarscope filter sets says he tunes the BFs individually to suit the etalons ... which are hand built not mass produced like the Lunt or Coronado units. I don't know exactly how he does it but I suspect it may have something to do with the excellent quality the Solarscope units tend to have (and maybe goes a fair way to explaining the weight of the price tag as well).


poita

Re: Large Aperture solar telescope using a modded Lunt etalon?

Post by poita »

Thanks so much for the detailed response Viljo (BTW, what is the correct way to pronounce your name?), I will have the Lunt on Monday so will take a look at it.

I am tempted to go straight to 150mm, my interest is in high-resolution work, hopefully my seeing is up to the telescope!

Can you remove the blocking filter from the B1200 diagonal? that would let you test if it is the mirror reflectivity or the filter itself causing the lower transmission.

If I was cynical I would say that the different transmission would be to try and hide the sweet-spot issues with the etalons in the Lunt, and would explain why Lunt users report that the sweet-spot is better on the Lunt vs the Coronado scopes.


User avatar
solarchat
Site Admin
Site Admin
Posts: 4359
Joined: Sat Feb 01, 2014 3:10 pm
Location: Atlanta, GA USA
Has thanked: 83 times
Been thanked: 1314 times
Contact:

Re: Large Aperture solar telescope using a modded Lunt etalon?

Post by solarchat »

the site qont stream videos no matter what size they are. I always upload my 2 to 10 GB videos to youtube and then embed them here.


Stephen W. Ramsden
Atlanta, GA USA
Founder/Director Charlie Bates Solar Astronomy Project
http://www.solarastronomy.org
poita

Re: Large Aperture solar telescope using a modded Lunt etalon?

Post by poita »

Anyone used a Celestron CR6? It seems a reasonable 150mm scope for the price.


User avatar
marktownley
Librarian
Librarian
Posts: 42270
Joined: Tue Oct 18, 2011 5:27 pm
Location: Brierley Hills, UK
Has thanked: 20424 times
Been thanked: 10243 times
Contact:

Re: Large Aperture solar telescope using a modded Lunt etalon?

Post by marktownley »

Anyone used a Celestron CR6? It seems a reasonable 150mm scope for the price.

I don't think you will go wrong with it. I think(?) Robert Arnold uses one for his PST mod....


Image
http://brierleyhillsolar.blogspot.co.uk/
Solar images, a collection of all the most up to date live solar data on the web, imaging & processing tutorials - please take a look!
brianb11213

Re: Large Aperture solar telescope using a modded Lunt etalon?

Post by brianb11213 »

If I was cynical I would say that the different transmission would be to try and hide the sweet-spot issues with the etalons in the Lunt, and would explain why Lunt users report that the sweet-spot is better on the Lunt vs the Coronado scopes.
An interesting theory - but it wouldn't explain why the "sweet spot" seems to be different between the pressure & tilt tuned versions of the Lunt scopes.

The theory could be tested by inserting a 2" Powermate between scope & diagonal. The altered convergence angle of the light reaching the blocking filter would alter any modification of the disc intensity using differential transmission in the blocking filter.


poita

Re: Large Aperture solar telescope using a modded Lunt etalon?

Post by poita »

I just meant that the sweet spot is not as noticeable when the image is darker.


DavidG
Ohhhhhh My!
Ohhhhhh My!
Posts: 109
Joined: Sun Jul 01, 2012 4:50 pm
Been thanked: 19 times

Re: Large Aperture solar telescope using a modded Lunt etalon?

Post by DavidG »

One thing that needs to be consider then choosing a scope to use for these mods is the fact that almost all of objectives designed for "typical" visual use, are designed to have the best correction around 550nm. Many times they have spherical aberration at 656nm. It's is usually not large but it is there. It can be very bad at 395nm thou. This is why Lunt using custom designed optics, be it singlets or achromats. You can purchase lenses that are design for using with HeNe lasers that maybe better corrected for use at 656nm. Another issue that is over looked is coma. The Sun is a very large object and again many objectives are not coma free so when your viewing the complete solar disk, the edge of the disk is located near the edge of field and this is were coma is it worst.

- Dave


Engineering = Take what you have and making what you need
poita

Re: Large Aperture solar telescope using a modded Lunt etalon?

Post by poita »

This is what I was wondering about, it seemed odd for Lunt to 'reinvent the wheel' without reason...
Hard to know what scope performs well at Ha, has anyone done any tests?

Would the lenses designed for lasers though be useful for a scope (I know nothing about laser stuff)


Photonist
Im an EXPERT!
Im an EXPERT!
Posts: 303
Joined: Sat Jan 07, 2012 5:57 pm

Re: Large Aperture solar telescope using a modded Lunt etalon?

Post by Photonist »

Poita,

I think I have to take the BF apart soon? But the weather person said it’s sunshine tomorrow, so… ?

Hard to explain the pronouncement of my name in text (glad I wasn’t named for example Yrjö) B)

Anyhow, the first three letters pronounce like when you say Wil(liam), then the j like you say the y in Y(ork) and the o like o in O(liver).

Viljo


poita

Re: Large Aperture solar telescope using a modded Lunt etalon?

Post by poita »

It will be interesting to see if it is the diagonal that is causing the lower light transmission or the blocking filter itself.
It does come apart fairly easily.

Thanks for the pronuciation guide, I like to get people's names correct :)

My name is Peter, but Australians tend to pronounce Peter as Poita, hence the handle here.


brianb11213

Re: Large Aperture solar telescope using a modded Lunt etalon?

Post by brianb11213 »

One thing that needs to be consider then choosing a scope to use for these mods is the fact that almost all of objectives designed for "typical" visual use, are designed to have the best correction around 550nm. Many times they have spherical aberration at 656nm. It's is usually not large but it is there. It can be very bad at 395nm thou.
Hmmm. Spherical aberration correction across the 400 nm - 700 nm "visual range" isn't usually an issue unless the objective is running at a fast focal ratio, it certainly shouldn't be an issue with the f/10+ objectives that modders will be interested in. I don't see any particular issues with my WO FLT 110 (at f/7) either ... the focus changes from the visual if you descend into the UV or ascend into the IR but there doesn't seem to be much difference in spherical correction. The resolution is still very good at 393 nm (CaK).

As for the off axis aberrations, with a conventional "thin" doublet or triplet, the limiting one is field curvature. Even so, experience is that a field flattener is not required to cover the whole solar disc.


colinsk

Re: Large Aperture solar telescope using a modded Lunt etalon?

Post by colinsk »

Anyhow, the first three letters pronounce like when you say Wil(liam), then the j like you say the y in Y(ork) and the o like o in O(liver).

Viljo

So we would say Wilya? Or is the w one of those consonants that English speaking people can't here? Sorry, I just like linguistics.


DavidG
Ohhhhhh My!
Ohhhhhh My!
Posts: 109
Joined: Sun Jul 01, 2012 4:50 pm
Been thanked: 19 times

Re: Large Aperture solar telescope using a modded Lunt etalon?

Post by DavidG »

[quote]One thing that needs to be consider then choosing a scope to use for these mods is the fact that almost all of objectives designed for "typical" visual use, are designed to have the best correction around 550nm. Many times they have spherical aberration at 656nm. It's is usually not large but it is there. It can be very bad at 395nm thou.
Hmmm. Spherical aberration correction across the 400 nm - 700 nm "visual range" isn't usually an issue unless the objective is running at a fast focal ratio, it certainly shouldn't be an issue with the f/10+ objectives that modders will be interested in. I don't see any particular issues with my WO FLT 110 (at f/7) either ... the focus changes from the visual if you descend into the UV or ascend into the IR but there doesn't seem to be much difference in spherical correction. The resolution is still very good at 393 nm (CaK).

As for the off axis aberrations, with a conventional "thin" doublet or triplet, the limiting one is field curvature. Even so, experience is that a field flattener is not required to cover the whole solar disc.

I recommend anyone interested in building narrow band solar telescope using an objective that was designed for visual wavelength to read the chapter on refractors in the new book "Telescopes Eyepieces and Astrographs" by Smith, Ceragiolli and Berry. Ceragioli does a great job in explaining spherochromatism which is spherical aberration as function of wavelength and coma and shows many examples of "typical" objectives designs and how much spherical aberration they have with different wavelengths. In a classic achromat there is usually only ONE wavelength that is perfectly corrected for spherical, that is usually around 550 nm. Most of the telescopes being discussed here for using with LUNT or PST Etalon are in the f/8 range were this problem is much worse then a type F/15 design.
A while back I showed a spot diagram of typical f/15 achromat at 395nm and the image was no longer diffraction limited, designer don't usually care about this wavelength. While spherical is not that bad at 656nm it is still typically not as well corrected as what it is at 550nm. The image is not going to be terrible, it is just not going to be as good as it could be. This is why LUNT uses custom designed achromats, designed for use at 656nm or 395nm. The same is also true of their designs that use singlets. The reason why they use achromats is that one can correct for both spherical aberration and coma at one wavelengths using spherical curves. It is also cheaper to make an achromat with four spherical surfaces then to use a singlet which would require an aspherical curve on one surface to be corrected for spherical aberation and coma at the same time. Don't be fooled that just becuase they are using an achromat and any old achromat will work as well as what they are doing.
Back in the good old days when Daystar was the only player, their ERF fixed many of these optical problems since besides reducing the energy flux that the filter saw it also stop the scope down to F/30. At F/30 and a typical aperture of 3" and under, many of these optical problems are greatly reduced.
When your using an f/8 refractor and apertures of 3" larger, designed for visual use and then use it with rear rear amounted Etalon, all those aberration are still there.

All the Best,
- Dave


Engineering = Take what you have and making what you need
User avatar
Merlin66
Librarian
Librarian
Posts: 3970
Joined: Tue Dec 13, 2011 1:23 pm
Location: Junortoun, Australia
Has thanked: 173 times
Been thanked: 615 times
Contact:

Re: Large Aperture solar telescope using a modded Lunt etalon?

Post by Merlin66 »

dave,
Thanks for the input and the reference to the new optics book.
I've been using:
http://www.telescope-optics.net/seconda ... matism.htm
as a good reference source.
I sort of understand the issue of spherical chormatic aberrations and in all the spectroscopes we use they all have achromat doublets as collimating/ imaging lenses. I agree the re-focus between the various wavelengths is an issue but I've never seen anyone record the loss of spectral resolution due to spherochromatic or chromatic coma aberrations.
(Chromatic coma does become a limiting factor [off-axis]when gratings are used "in the converging beam")
Obviously need to do more reading.....


"Astronomical Spectroscopy - The Final Frontier" - to boldly go where few amateurs have gone before
https://groups.io/g/astronomicalspectroscopy  
http://astronomicalspectroscopy.com
"Astronomical Spectroscopy for Amateurs" and
"Imaging Sunlight - using a digital spectroheliograph" - Springer
Photonist
Im an EXPERT!
Im an EXPERT!
Posts: 303
Joined: Sat Jan 07, 2012 5:57 pm

Re: Large Aperture solar telescope using a modded Lunt etalon?

Post by Photonist »

[quote]Anyhow, the first three letters pronounce like when you say Wil(liam), then the j like you say the y in Y(ork) and the o like o in O(liver).

Viljo

So we would say Wilya? Or is the w one of those consonants that English speaking people can't here? Sorry, I just like linguistics.


Colin,

He he, now I probably can’t pronounce Wilya :?

Actually the Finnish language has no W, so William was a poor example from me, perhaps Vi(ctor) would be better? The last letter o should be like the o in o(h), short, open and without any hint of u.(I know nothing about phonetics, grammar or linguistics, I just learned my five lang by ear) :)

Well, how far can we drift from the original Lunt 60 mod topic, sorry Ken and Steven and all B)

Viljo


poita

Re: Large Aperture solar telescope using a modded Lunt etalon?

Post by poita »

In trying to mod the Lunt BF the first thing I would check is the reflectivity of the mirror. I was told before production started that three mirrors were going to be made. 30%, 40% and 50% reflective and the choice of which one to use would be made on a etalon to etalon basis. I don't have any idea if this plan was ever implemented but I do have reason to suspect that the diagonal mirror is not designed to be 90% reflective.

I found this reply from lunt when repairing a faulty B600 interesting when the owner asked why the focus position changed when the filter was returned.

In your case I also replaced the internal mirror with a LWP (Long Wave Pass filter). This should help with contrast. The LWP has a different backing plate design which changes the focal point of the system slightly.


poita

Re: Large Aperture solar telescope using a modded Lunt etalon?

Post by poita »

Interestingly, the standard Lunt 60 focuser fits perfectly inside my ED80 scope with no need for extra spacers, so I'm going to give that a go while waiting for the bigger donor scope.


User avatar
Merlin66
Librarian
Librarian
Posts: 3970
Joined: Tue Dec 13, 2011 1:23 pm
Location: Junortoun, Australia
Has thanked: 173 times
Been thanked: 615 times
Contact:

Re: Large Aperture solar telescope using a modded Lunt etalon?

Post by Merlin66 »

Peter,
That sounds interesting...as long as you can maintain the "majic" positioning of the Lunt shoulder relative to the the prime focus, you should be OK.....


"Astronomical Spectroscopy - The Final Frontier" - to boldly go where few amateurs have gone before
https://groups.io/g/astronomicalspectroscopy  
http://astronomicalspectroscopy.com
"Astronomical Spectroscopy for Amateurs" and
"Imaging Sunlight - using a digital spectroheliograph" - Springer
Allcart
Im an EXPERT!
Im an EXPERT!
Posts: 496
Joined: Wed Jan 09, 2013 1:59 pm

Re: Large Aperture solar telescope using a modded Lunt etalon?

Post by Allcart »

Interestingly, the standard Lunt 60 focuser fits perfectly inside my ED80 scope with no need for extra spacers, so I'm going to give that a go while waiting for the bigger donor scope.

Well this was a really interesting read. So much info its hard to digest.

I would be interested in an update Peter. How did the LS60 work in your ED80?


Allan
Bratislav

Re: Large Aperture solar telescope using a modded Lunt etalon?

Post by Bratislav »

Hello everyone! I've just joined the SolarChat after lurking for a long time.

I'd like to ask possibly a silly question about the Lunt mod discussed here (which I intend to replicate in the near future first with a 100mm APO and then grind a larger, 5 or 6" singlet).
Why is an extra ERF filter (as in Baader D-ERF) needed if Lunt 60 has already an effective ERF coating on the diverging lens ? Considering that donor scope will typically be much slower f-ratio than original singlet in the Lunt 60, energy density should be (much) lower, even if the lens is larger in absolute terms. In other words, if ERF coating works for LS60T, why shouldn't it work for a modded scope? The only thing I can think of is heat buildup (diverging lens will dissipate heat inside tube, affecting the seeing, while ERF up front removes the heat before entering.
What am I missing here?

Thanks,
Bratislav


User avatar
swisswalter
Way More Fun to Share It!!
Way More Fun to Share It!!
Posts: 17948
Joined: Thu Feb 06, 2014 4:28 am
Location: Switzerland
Been thanked: 1 time
Contact:

Re: Large Aperture solar telescope using a modded Lunt etalon?

Post by swisswalter »

Hi Bratislav

welcome to that wonderful TOS free site, have fun and mod ;) I would do it with an ERF for safety reasons


Only stardust in the wind, some fine and some less fine scopes, filters and adapters as well. Switzerland 47 N, 9 E, in the heart of EUROPE :)

from 7 am - 7 pm http://www.nanosys.ch

from 7.01 pm - 6.59 am http://www.wastronomiko.com some times vice versa ;)
User avatar
Merlin66
Librarian
Librarian
Posts: 3970
Joined: Tue Dec 13, 2011 1:23 pm
Location: Junortoun, Australia
Has thanked: 173 times
Been thanked: 615 times
Contact:

Re: Large Aperture solar telescope using a modded Lunt etalon?

Post by Merlin66 »

I think the use of the up front ERF (on a 150mm aperture!) was "belt and braces" protection.
The energy on the front lens (albeit an ERF)of the etalon assembly is (150/60)^2 = x6 times the original design....


"Astronomical Spectroscopy - The Final Frontier" - to boldly go where few amateurs have gone before
https://groups.io/g/astronomicalspectroscopy  
http://astronomicalspectroscopy.com
"Astronomical Spectroscopy for Amateurs" and
"Imaging Sunlight - using a digital spectroheliograph" - Springer
Bratislav

Re: Large Aperture solar telescope using a modded Lunt etalon?

Post by Bratislav »

I think the use of the up front ERF (on a 150mm aperture!) was "belt and braces" protection.
The energy on the front lens (albeit an ERF)of the etalon assembly is (150/60)^2 = x6 times the original design....

I agree about "belt and braces", you can never be too sure. But I won't be spending too much time risking my eyeballs (a camera will take that place), that is why I ask this question. I just want to be absolutely sure I understand how LS60 works before embarking on the mod.
BTW, energy is indeed (150/60)^2 higher, but is also spread across the (1200/280)^2 area, so energy density should be much lower. ERF should really work in energy per area, if I'm not mistaken.

Thanks,
Bratislav

PS - the deep blue coating on LS60's front lens should be just an antireflection coating, right? In the very unscientific experiment of using just a front lens to focus the Sun onto a piece of paper, it burned a nice clean hole in it. The image was quite yellow though, so perhaps some UV rejection is done by front lens as well


User avatar
Merlin66
Librarian
Librarian
Posts: 3970
Joined: Tue Dec 13, 2011 1:23 pm
Location: Junortoun, Australia
Has thanked: 173 times
Been thanked: 615 times
Contact:

Re: Large Aperture solar telescope using a modded Lunt etalon?

Post by Merlin66 »

Bratisav,
The size of the beam at the internal etalon should be very similar, so the energy will be higher?
Re the objective IMHO it's just a AR coating....


"Astronomical Spectroscopy - The Final Frontier" - to boldly go where few amateurs have gone before
https://groups.io/g/astronomicalspectroscopy  
http://astronomicalspectroscopy.com
"Astronomical Spectroscopy for Amateurs" and
"Imaging Sunlight - using a digital spectroheliograph" - Springer
Bratislav

Re: Large Aperture solar telescope using a modded Lunt etalon?

Post by Bratislav »

Hmmm... I'll have to think about this. Maybe a raytrace model in Zemax would help, but yes, at a first glance it indeed sounds that in telecentric part of the telescope energy density would be proportional to entrance pupil size, rather than f-ratio. In that case ERF coating will definitely be exposed to much higher energy per square mm. Good point!

The objective definitely rejects good portion of a blue part of the spectrum, it is quite obvious that image (before it gets blindingly bright in focus) is very warm (yellow). Perhaps they use it to reject UV as well...


colinsk

Re: Large Aperture solar telescope using a modded Lunt etalon?

Post by colinsk »

Telecentric systems are dependent on the focal length. Collimated systems are dependent on the sum of the focal lengths of the objective and the collimation lens being set to the spacing of the lenses. Vignetting is a crucial problem in changing the objective diameter.


Allcart
Im an EXPERT!
Im an EXPERT!
Posts: 496
Joined: Wed Jan 09, 2013 1:59 pm

Re: Large Aperture solar telescope using a modded Lunt etalon?

Post by Allcart »

Finally, here is my Lunt/C6-R 152mm mod project. It was built using information in this valuable resource and based on David Cortner's design. It is not completely finished yet. I have a bit of flocking to do and a few more moddifications to make it a modular system. At the moment it is a complete scope and awkward to handle.
Initially I could not aquire focus at the spacings worked out in this topic and found that I needed to move things slightly, but the mesurements need to be verified and more testing needs to be done.
I removed the focuser from the large aperture scope and mounted the ERF in a cell in its place. I have found no internal reflections at all, and the view is superb. There is a sweetspot as expected, but it can be moved around the disc to view various features.
Having an open space between the Lunt and main scope does not seem to effect the performance, but I shall cover the gap and see if it makes any improvement to contrast etc.

Some pics;
Basic layout of the Lunt section. The ERF is housed in the black portion of the tube and held in place with a tight foam cell. The black part is the piece I cut off for the pst mod and provided a convenient place to put the ERF.

IMG_3125 by allcart2, on Flickr


IMG_3126 by allcart2, on Flickr

The full scope. It weighs just over 30lbs fully assembled and is 1500mm long. The original scope was black, but has a new paint job to match the Lunt.

IMG_3122 by allcart2, on Flickr


Allan
User avatar
swisswalter
Way More Fun to Share It!!
Way More Fun to Share It!!
Posts: 17948
Joined: Thu Feb 06, 2014 4:28 am
Location: Switzerland
Been thanked: 1 time
Contact:

Re: Large Aperture solar telescope using a modded Lunt etalon?

Post by swisswalter »

Hi Allan

what a great mod. I would like to see results in form of pics ;-)


keep on the good work


walter


Only stardust in the wind, some fine and some less fine scopes, filters and adapters as well. Switzerland 47 N, 9 E, in the heart of EUROPE :)

from 7 am - 7 pm http://www.nanosys.ch

from 7.01 pm - 6.59 am http://www.wastronomiko.com some times vice versa ;)
Allcart
Im an EXPERT!
Im an EXPERT!
Posts: 496
Joined: Wed Jan 09, 2013 1:59 pm

Re: Large Aperture solar telescope using a modded Lunt etalon?

Post by Allcart »

Thanks Walter, I have only just had the first light with it, but I am very pleased with how it performed. I posted a couple of pics in the solarcahat forum.

http://solarchat.natca.net/index.php/en ... -mod#70250


Allan
Allcart
Im an EXPERT!
Im an EXPERT!
Posts: 496
Joined: Wed Jan 09, 2013 1:59 pm

Re: Large Aperture solar telescope using a modded Lunt etalon?

Post by Allcart »

Ok, so this could be the last element of my Large Aperture Lunt modifications. I purchased a LS60f DS filter to double stack on my LS60T. Today I decided to use it to double stack my 120mm lunt mod. I can use the same cradle as I use with my Lunt C6 mod but I just have to change the position of the lunt etalon to suit the smaller scope.
I don't have an adaptor plate for the DS filter, so I made one out of an aluminium camping dish and some toilet seat fittings. Total cost.......£0.00 Here it is on the scope.


IMG_3148 by allcart2, on Flickr

Full scope.


IMG_3150 by allcart2, on Flickr

I'm not sure if this is a step too far as I have just reduced my aperture back to 60mm with a focal length of about 1250mm. I could probably get the same results with a barlow on the LS60T. Anyway, here are some first light pics.


Marching proms 7-7-2013 by allcart2, on Flickr

An attempt at a 6 pane mosaic with a DMK31.


AR1785 mosaic 7-7-2013 by allcart2, on Flickr

This is very near to what I see visually.


AR1785 DMK41-120 DS HA by allcart2, on Flickr


Allan
User avatar
swisswalter
Way More Fun to Share It!!
Way More Fun to Share It!!
Posts: 17948
Joined: Thu Feb 06, 2014 4:28 am
Location: Switzerland
Been thanked: 1 time
Contact:

Re: Large Aperture solar telescope using a modded Lunt etalon?

Post by swisswalter »

Hi Allan

a fine mod and good first results


Only stardust in the wind, some fine and some less fine scopes, filters and adapters as well. Switzerland 47 N, 9 E, in the heart of EUROPE :)

from 7 am - 7 pm http://www.nanosys.ch

from 7.01 pm - 6.59 am http://www.wastronomiko.com some times vice versa ;)
Post Reply